This invention relates to glass melters and more particularly to a glass melter having an outlet portion formed as a heating device for controlling the outlet flow rate of molten glass from the melter.
Glass melters, especially those used for the production of glass fibers, generally operate continuously, twenty-four hours a day, seven days a week. However during such continuous operation, the output flow of molten glass from the melter must be varied in accordance with fluctuating demands beyond the melter, as for example, at a forehearth.
The forehearth, in a glass fiber forming operation, normally receives molten glass from a refiner that is in series with the melter. The refiner receives the entire output of molten glass from the melter and passes such output onward to the forehearth. The forehearth thus accepts a continuous outlet or output flow of molten glass from the melter and distributes such output to a series of glass fiber forming bushings.
Each bushing in series of glass fiber forming bushings contains a plurality of orifices from which the glass fibers are drawn, as disclosed in U.S. Pat. No. 4,146,375. Glass fibers or filaments from the bushing are gathered into a strand that is wound around a collet.
Generally, the outlet flow from the glass melter is capable of fulfilling the demand for molten glass by a predetermined maximum number of glass fiber forming bushings that each have a set number of potentially operable glass fiber forming orifices. However, since the orifices in a glass fiber forming bushing are relatively small, one or more such orifices may become clogged or otherwise fail to operate, thereby decreasing the glass fiber output of one of the bushings in a series.
Thus the output from the glass fiber forming bushings can fluctuate due to operational problems at one or more orifices. Such problems can last for varying time periods. On occasion, it may be necessary to shut down a complete bushing, resulting in a distinct decrease in the amount of molten glass needed by the forehearth. Therefore it is necessary to decrease the outlet flow from the glass melter to compensate for any decreases in demand at the forehearth.
Since servicing of an inoperable bushing is not always immediate, the decreased demand for molten glass at the forehearth may exist for a prolonged period of time. Furthermore there are instances when more than one bushing, in a series of several bushings, must be cut off from receiving a flow of molten glass.
The withdrawal of more than one bushing from the fiber forming operation usually results in a substantial curtailment of demand for molten glass at the forehearth in comparison to the maximum potential demand when all of the bushings are operating at their rated capacity.
The magnitude of any change in demand for molten glass by the forehearth must therefore be recognized and responded to at the glass melter, such as by a flow control means incorporated in the melter. Ideally, the flow control means for a glass melter should be capable of accurately responding to wide variations in demand for molten glass at operating stations in series with the melter.
One known approach for controlling the flow of molten glass from a glass melter to a forehearth, includes the use of a needle or plunger type valve as shown in U.S. Pat. Nos. 3,580,976 and 3,659,029. Flow control by such needle or plunger valves is accomplished by varying the position of a tapered shaft in a fixed diameter tube to restrict or vary the rate of flow of molten glass through the tube.
The known needle valve arrangements require use of exotic and extremely expensive materials, such as molybdenum or molybdenum alloys, which have a limited tolerance for withstanding the hostile environment in the glass melter without contaminating the molten glass. Eventually however a wearing away of the needle valve parts occurs, which adversely affects the flow control function of such valve. It thus becomes necessary to shut down the glass melter for several days to repair and/or replace the worn out needle valve assembly. Since the glass melter is most efficient when it operates continuously, any shutdown is extremely costly.
At present the needle-type valve arrangement for controlling the outlet flow of molten glass from a melter has provided the only feasible solution to the problem of dealing with a wide range of demand for molten glass by the forehearth that is in series with a glass melter.
It is thus desirable to provide a flow control system for a glass melter that furnishes a wide range of control without requiring a flow restricting valve for accomplishing such control.